The Physics of the Invisible Box Challenge


People dance. It is what they do. Everybody at all times needs to get some cool new dance transfer. First there was the electrical slide. Yeah, that was cool—however then there was the moonwalk. That was actually cool. And now we’ve got the invisible field. OK, perhaps it is not precisely a dance transfer, however extra like a trick. The fundamental thought of this transfer is to make it seem to be the dancer is stepping on a block—a block that is invisible. It is a powerful transfer, however how does it work?

Let me begin with an evaluation of the movement. I’ll use a video from twitter together with video evaluation methods. Here’s a plot of the vertical place of the lady’s proper foot (the one which steps on the “the field”) throughout this transfer.

OK, she does a reasonably good job with that foot. Discover that it has a reasonably fixed vertical place whereas she “steps” on the invisible block. In reality, the foot is stationary for nearly zero.2 seconds. That may not seem to be a very long time, but it surely’s lengthy sufficient to look superior.

Since (SPOILER ALERT) there is not actually a field there, the lady is definitely within the air for just a little little bit of time with solely the gravitational pressure performing on her. Which means she could possibly be thought of a kind of projectile movement with a continuing vertical acceleration of -9.eight m/s2. However wait! If she is accelerating down, how does her foot keep stationary? The important thing right here is that her middle of mass accelerates down, however she will be able to make part of her (like her foot) keep stationary.

I might in all probability estimate her mass distribution to calculate her middle of mass in every body—however I’ll as a substitute simply plot the vertical movement of some of her physique components (ft, arms, torso). Right here is the vertical place for all of those components.

From this plot, it is best to discover a few issues. First, the underside foot truly would not transfer up or down that a lot in comparison with the remainder of the physique. Second, the physique appears to maneuver in method with a extra fixed acceleration—such as you would anticipate for projectile movement. Do her arms have a big affect on the transfer? Possibly, it is troublesome to inform.

However I am not solely right here for the physics evaluation. No, I am right here that will help you pull this off. If you wish to do that dance transfer, there are two issues that it’s essential know—however keep in mind, I am a physicist and never a dancer. The upper you soar, the longer you may maintain your foot in place. Since this soar is rather like projectile movement, you wish to soar as excessive as doable. Nevertheless, the “grasp time” for a soar is proportional to the sq. root of the peak. For a half a second grasp time, you would want to leap up 30 centimeters. That may not appear very excessive—however that’s the peak of your middle of mass. It is more durable than it appears.

The opposite essential factor to think about is the leaping pressure. So as to step on the invisible field, one in every of your legs would not actually do something besides to “not transfer”. Which means you need to soar over the invisible field utilizing just one leg. That isn’t a very simple activity. However I suppose if it was straightforward, everybody would do it and it would not be so cool.

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